In a computer-to-plate (CTP) printing system, pages to be rendered are represented in a page description language format generated, for example, by a desktop publishing system. A widely used page description language is the PostScript.RTM. language. (PostScript.RTM. is a registered trademark of Adobe Systems, Inc.) The PostScript.RTM. language is fully described in the PostScript.RTM. Language Reference Manual, Second Edition (1990), from Adobe Systems, Inc., which is incorporated herein by reference.
The pages to be rendered may be described in a PostScript.RTM. file (or in a plurality of PostScript.RTM. files), wherein each PostScript.RTM. file contains all the information, definitions and instructions to fully describe and render all the pages in the file. Although it is not a restriction of the PostScript.RTM. language, Adobe Systems, Inc. (the creator of the PostScript.RTM. language) recommends that the individual pages described in the PostScript.RTM. file be completely independent of each other. In reality, however, complete page independence is often an impractical restraint. For example, if several different non-resident fonts are used on every page, complete page independence would require the font definitions to be repeated for each page. Because of this impracticality, any individual page in the PostScript.RTM. file often contains definitions and instructions that are needed to describe and render another page in the file.
For efficiency in printing, selected individual pages from a PostScript.RTM. file are often imposed (or positioned) onto a flat or plate so that paper printed from the flat can be cut and folded into book segments or signatures with the pages in the correct order. Imposition is also used when producing film from which a plate will be made. It is desirable to send only the selected pages to be rendered on the flat to an imagesetter or platemaker for physical rendering of the selected pages. This requires, however, that the pages described in the PostScript.RTM. file are completely independent of each other. Therefore, lack of page independence in the PostScript.RTM. file creates problems when imposing selected pages on a flat because instructions and definitions affecting these selected pages to be imposed on the flat but contained in a page description for a page not included on the flat may be lost. Further, the lack of page independence may prohibit other applications, such as, for example, printing only selected pages in the PostScript.RTM. file or printing alternating pages with different offsets and margins for binding.
Several approaches have been developed to overcome these problems and to achieve page independence in a PostScript.RTM. file in order to allow for effective imposition. The first approach is to pre-process the PostScript.RTM. file. This approach has several disadvantages. It is inefficient, and a pre-processor must be developed and/or updated as the PostScript.RTM. generating application or PostScript.RTM. file is developed and/or updated. The second approach is to remove all definitions from the PostScript.RTM. file by reducing all procedures to pure instructions. Although several commercial packages are available to implement this second approach, none of these packages are specifically designed for use in the graphics art field, which generally involves high resolution output devices. Consequently, this type of processing is slow and does not work adequately with high resolution output devices since approximately one-third of the PostScript.RTM. instructions, including some parameters for screening images, are lost. Further, device-dependent procedures and conditionally executed code are sometimes reduced to incorrect instructions.
Thus, there remains a need for a method of imposing and rendering selected pages which are described in a PostScript.RTM. file onto a flat without performing any preprocessing operations to try to achieve page independence in the PostScript.RTM. file. There also remains a need for a method that allows flexibility for use with non-imposition applications, such as printing only selected pages from the PostScript.RTM. file.